Field of Invention
This invention relates to finding the location of a fault on a power line.
Prior Art
Electric utilities must monitor their transmission lines to insure their proper operation and prevent damage to their systems or customers equipment. Power outages can result from many different causes, some internal to the system such as equipment failure others from external causes such as a tree falling on a power line. In all cases the utility must find and repair the damage as rapidly as possible. Often the problem is quickly locating the point of failure. Utilities use many different devices to detect abnormal conditions in the power system. Devices currently in use measure the voltage and current on the power lines to insure if they are within normal limits. Should these devices detect a fault then a circuit breaker is opened and the line is protected from overload.
Some types of faults such as noise on the power lines are not detected by these devices. This noise may not indicate an immediate failure. The cause of the noise may not indicate a failure at all. The noise can go on for years. Such noise can emanate from arcing around mounting bolts and fixtures. These parts are very heavy and are not seriously damaged by the arcing. However, noise often indicates a fault with the equipment and can result in an eventual failure.
This noise, even if it is not a problem for the utility, can be a problem for other people in the area. It can disrupt radio communication, television reception, and other equipment. Above certain limits this noise can violate government restrictions on unintended radiation emissions. This can result in fines.
Noise emissions can be located using special wide bandwidth RF receivers and directional antennas such as equipment from Exacter Inc or Radar Engineers. With these devices a person must drive around searching for the strongest reception on his receiver. The receiver allows the operator to listen to the noise. Sometimes the noise has audio characteristics that can give a clue as to the type of fault. This is tedious, time consuming, and cannot be automated.
U.S. Pat. No. 7,777,676 describes a system for locating lightning strikes by using the time difference as measured by receivers at multiple points. This system is synchronized using GPS receivers. However this system depends on the lightning strike having a known waveform. This known waveform must have large enough amplitude to stand above the background noise. It will not work on signals buried in the noise. This patent, while not related to power lines, does describe relevant prior art.
U.S. Pat. No. 2,717,992 discloses a system where the impulse resulting from a transient fault is detected at each end of the power line. The two detectors communicate and produce graphical output that indicates the position of the fault on the power line. The graph shows the relative timing of the impulse. The difference in the arrival times is proportional to the distance to the fault.
U.S. Pat. No. 3,462,681 discloses a system that detects an electrical fault by sensing the disturbance wave as it travels to each end of the cable and senses the disturbance as it passes detectors connected to the ends of the cables and calculates the time difference of the arrival of the disturbance at the sensing points and thereby determines the location of the fault. This is a similar method to patent '992 in that it times the arrival of the impulse resulting from a fault.
An early U.S. Pat. No. 2,493,800 discloses a technique where a high voltage is applied to the power line. This voltage is high enough to cause a voltage breakdown. The timing of the resulting wave from the breakdown is measured at each end of the line. The result is shown on a CRT display as a pattern that indicates the position of the breakdown.
U.S. Pat. No. 6,822,457 discloses a system for locating faults on a power line but includes transmitters that apply a test signal to the line to determine its characteristics. This measures the propagation delay of the power line. This is an important factor in the calculation of the exact position of a fault. However, it can be impractical to inject a test signal on an energized line. Additionally, such test signals will also radiate and potentially disrupt radio communications.
U.S. Pat. No. 5,608,328 does attempt to locate an arcing fault. It does this by detecting the polarity of a high current fast rise time pulse. A portable detector is moved along the cable while the fault is occurring. Then the polarity of the pulse changes then the fault has been found. This system is not suitable for permanent installation and requires a fault pulse to work.
U.S. Pat. No. 7,577,535 describes an apparatus capable of determining the direction of a fault relative to the apparatus. It is a system that uses Fourier analysis to extract the frequencies of interest. It is useful in locating arcing faults that have not yet resulted in a system outage. However it is not able to determine the distance to the fault rather it is a mobile device that locates the fault by moving along the path of the power line.
U.S. Pat. No. 7,869,810 issued Jan. 11, 2011 to Agilent is written toward locating signal sources in an office environment. This claims an iterative process of adjustments to the map to improve its quality in the presence of multipath reflections. Since they are searching for an emitter operating in a known frequency band they cannot choose an optimal band to reduce such reflections.
U.S. Pat. No. 8,049,668 issued Nov. 1, 2011 to Agilent attempts to locate a device emitting an RF signal, likely a WiFi device or cell phone, in an office environment by calculating weights and residuals. The estimate for the location of the emitter is updated in an iterative manner that improves the estimate.
U.S. Pat. No. 8,098,200 issued Jan. 17, 2012 to Agilent attempts to locate a RF emitter using a likelihood function. Such an emitter is likely a WiFi device or cell phone. The likelihood value is determined for each position on a grid. The position where the likelihood value is the greatest is the estimated position of the emitter.